RISC-V: Initial DTS for Microchip ICICLE board

Add initial DTS for Microchip ICICLE board having only
essential devices (clocks, sdhci, ethernet, serial, etc).
The device tree is based on the U-Boot patch.

https://patchwork.ozlabs.org/project/uboot/patch/20201110103414.10142-6-padmarao.begari@microchip.com/



Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>